Among the available common engineering alloys, magnesium alloys have the highest strength-to-weight ratio. Use of magnesium alloys in automobiles has been increasing due to the need of increasing fuel economy, reducing pollution and lessening our dependence on petroleum. Recently, several new applications in various parts of vehicles have been developed, including oil pans, gearbox housings, and radiator support assemblies.
However, use of magnesium alloys for vehicle powertrain systems, such as engine blocks, has been quite limited to date. One limitation on the use of magnesium alloys in powertrain systems is their poor corrosion resistance, especially when they are in contact with the water/glycol based heat transfer fluids (coolants) commonly used in vehicle cooling systems.
The corrosion inhibitors currently used in water/glycol based heat transfer fluids are formulated with specific blends of silicates, nitrites, mono- or di-carboxylic acids or their salts (such as C4-C18 mono- or di-carboxylic acids, and benzoates), molybdates, nitrates, phosphates, phosphonates, and/or borates to provide corrosion protections for various metals in the cooling systems. Although many of these inhibitors can provide satisfactory corrosion protection for various metallic components used in vehicle cooling systems (including aluminum, cast iron, steel, copper, brass, and solder), corrosion protection for magnesium alloys is poor. Corrosion rates of the magnesium alloys are especially high when the alloys are in galvanic contact with other metals and/or at high operating temperature (e.g., >90° C.), and in contact with heat transfer fluids not designed for use with magnesium alloys.
Thus, there is a need for new and more effective corrosion inhibitor compositions and corrosion inhibiting heat transfer fluids for use in vehicle cooling systems containing magnesium or magnesium alloys.